Sintering of iron-aluminum base powders



3,009,809 SINTERING OF IRON-ALUMINUM BASE POWDERS Joseph Neri, Jr., Towson, Md., assignor, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Feb. 27, 1959, Ser. No. 795,905

11 Claims. (Cl. 75201) This invention relates to iron-aluminum base alloys containing up to about 11.0% aluminum, and to a method of preparing same by power metallurgy methods.

Heretofore, iron-aluminum base alloys have been prepared by melting together the constituent metals and casting. Attempts to sinter powders containing principally iron and aluminum have been unsuccessful, primarily because of the very tenacious oxide coating on the aluminum particles. Since oxide-free aluminum powder is relatively costly and requires special handling, it is very desirable to work with commercially available material.

A purpose, then, of this invention is to provide a method of preparing iron-aluminum base alloys from commercial powders, using standard powder metallurgy techniques. In this manner the advantages of powder methods may be applied to iron-aluminum metallurgy, such as the fabrication of intricate parts which cannot be cast.

In accordance with the present invention an iron-aluminum base powder is prepared in which the aluminum content is between about 4.0% and 11.0% by weight. Suitable compositions are disclosed in the copending patent application of Ida et al., Serial No. 687,606, filed October 2, 1957, now US. Patent No. 2,941,883, which discloses the combination with the iron-aluminum base of certain additives for imparting desired characteristics of the final alloy. There can be added a total of up to about 15.0% by weight of one or more elements which are normally alloyable with the primary constituents, iron and aluminum. With specific reference to nuclear reactor applications, for example, cladding of reactor fuel elements, the metal additive must have a relatively low cross-section for thermal neutrons and also operate to increase the strength or corrosion resistance of the alloy. Of particular utility in this regard are the elements molybdenum, niobium, titanium, beryllium, nickel, chromium and zirconium.

The powder to be sintered, consisting essentially of about 4.0% to 11.0% of aluminum, up to 15.0% of the aforementioned additives, and the balance iron is admixed with silicon in an amount not less than about 10.0% of the weight of aluminum present and tin in an amount not less than about 0.5% of the total weight of the admixture. It will be readily appreciated that certain incidental impurities will also be found in the powder mixture such as are ordinarily associated with the con- 1 stituent elements after refining. It is preferred, however,

that the carbon level be maintained below about 0.15 by weight of the total.

The upper limits of the concentrations of tin and silicon in the alloy are determined by considerations of strength, corrosion resistance, ductility and other properties. Depending upon what alloy characteristics are desired, these elements may be added up to saturation.

The constituents of the alloy to be made may each be initially in elemental powder form. It is preferred, however, to add silicon as powdered aluminum-silicon eutectic alloy, which contains 11.6 weight percent silicon. The powder mixture may also be derived from particles of suballoys prepared by standard melting techniques. A convenient practice is to prepare a master alloy of the major components, for example, iron and aluminum, or iron, aluminum and chromium, which may be used as 3,09,09 Patented Nov. 21., 1961 desired to make master alloy powder. It is apparent of course, that many other alloy combinations of the constituent elements in the starting powder mixture are possible. It is essential, however, that both the silicon and tin present in the powder mixture be substantially free of oxide over that amount of oxide usually associated with these elements when exposed to the atmosphere.

It has been found that both tin and silicon must be present in the iron-aluminum base powder mixture to induce good sintering. Although either of these elements separately will produce some sintering action, the resulting product has poor ductility and tends to fracture during rolling. It is believed that the tin and silicon cooperate to remove the oxide coating on the metal particles, but the mechanism by which this reaction occurs is not presently understood.

The powder mixture is thoroughly blended so as to provide a uniform mixture. It is preferred to use powders having an average particle size of about 325 mesh, but other suitable particle sizes may be used. After blending, the powder mass is pressed into shape. Depending upon what as-sintered density is desired, the pressing pressure is varied from a minimum of approximately 10 tons per square inch to 30 tons per square inch or more. Dense compacts have usually been formed at pressure of 30 t.s.i., but satisfactory densification is effected at a lower pressure, 20 t.s.i., if the primary components are present in the initial powder mixture as unalloyed elements. It has been observed that powder blends of the constituent elements generally sinter more readily than particle mixtures in which the iron and aluminum are alloyed. As a result, powders containing particles of a sub alloy, such as 7Al-5Cr88Fe, 5Al--8Cr--87Fe, or

are compacted under higher pressures, that is, 30 t.s.i. or more.

Sintering is accomplished in at least two heating steps, and in a protective atmosphere to prevent oxidation of the powder particles. The compacted powder mixture is first heated to between about 500 C. and 600 C. for approximately /2 to 1 hour. The temperature of the compact is then raised to about ll50 to 1300 C. and maintained at that temperature for an interval of 1 to 2 hours, during which time sintering is completed.

In order to provide a clearer understanding of the present invention, several specific examples are provided.

Example I A powder mixture was prepared fro-m particles of the following materials:

This mixture was pressed at 20 t.s.i. into a briquet 1.250 x 4.50 x 0.125 inch, heated in a dry hydrogen atmosphere at 600 C. for 1 hour, and sintered at 1200 C. for 2 hours. Excellent sintering was obtained. The sample was then reduced by cold rolling from 0.125 inch to 0.025 inch in eight passes and annealed at 1000 C. for about 30 minutes. The annealed sheet could be bent back upon itself without fracturing. Microscopic examination showed that the oxide phase had been well dispersed.

Ductility of the final roller article is generally improved, if the original pressed compact is progressively sintered in three stages consisting of approximately one hour at 500 0., two hours at 1050 C. to 1150 C., and one hour at 1300 C. to 1375 C. Also, improved microstructures are observed in rolled alloy strips that have been given intermediate anneals at about 1000 C. for approximately 15 minutes between passes.

By way of comparison with the foregoing example, a powder mixture containing essentially by weight percent 93.0% of carbonyl iron and 7.0% of aluminum was pressed at 30 t.s.i. Using the heating schedule of Example I, an extensively laminated product was obtained, which exhibited very little sintering action. The addition of tin or silicon to the mixture of iron and aluminum particles brought about a marked improvement in assintered and microstructure. Nevertheless, it was found that the grains were in an embrittled condition, which caused fracturing during subsequent rolling operations.

A powder blend was prepared from particles of the following materials:

Example 11 Weight percent Fe (carbonyl) 87 AlqSi eutectic alloy 6 Cr Mo 1 Sn 1 The powder mixture was pressed at 20 t.s.i. and sintered in dry hydrogen 1 hour at 600 C., two hours at 1075 C. and one hour at 1350 C. The resulting compact was cold rolled from a thickness of 0.125 to 0.040" and annealed for /2 hour at 1000 C. Tensile specimens were cut from the sheet and pulled at room temperature. These specimens were found to have an average tensile strength of 80,000 p.s.i., with an elongation of 17%, and could be bent readily.

While a detailed description of the present invention and a preferred embodiment thereof have been provided, it will be apparent to those skilled in the art of powder metallurgy that various changes and modifications may be made therein without departing fro-m the spirit or scope of the invention.

I claim:

1. A method of sintering a superficially oxidized ironaluminum base power mass consisting essentially by weight of between about 4.0% to 11.0% 'of aluminum, and the balance iron, said method comprising the steps of uniformly admixing with said powder mass silicon in an amount such that the weight ratio of silicon to aluminum is at least 0.1, and tin in an amount equal to at least 0.5 by weight of the total mixture, and progressively heating the resulting powder mixture in a non-oxidizing environment from a temperature between about 500 C. and 600 C. maintained for not less than about 1 hour, to a terminal temperature between about 1300" C. and 1375 C. maintained for not less than about one hour.

2. The method of claim 1 wherein said heating of said powder mass includes an intermediate heating interval at a temperature of about 1050 C. to 1150". C. for not less than about 2 hours.

3. The method of claim 1 wherein said silicon is added to said powdermass inthe form of an aluminum-silicon alloy.

4. A method of sintering a superficially oxidized ironaluminum base powder mass containing essentially by weight about 4.0% to 11.0% of aluminum, a total of not more than about 15.0% of at least one metal additive selected from the group consisting of Mo, Nb, Ti, Be, Ni, Cr, and Zr, the balance being iron, said method comprising the steps of uniformly admixing with said powder mass silicon in an amount not less than about 10.0% by weight of the aluminum present, and tin in an amount equal to at least about 0.5% by weight of the total mixture, and progressively heating the resulting mixture in a non-oxidizing environment from a temperature between about 500 C. and 600 C. maintained for not less than about 1 hour, to a temperature between about 1300 C. and 1375 C. maintained for not less than about 1 hour.

5. The method of claim 4 wherein said heating of said powder mixture includes an intermediate heating interval at a temperature of about 1050 C. to 1150 C. for not less than about 2 hours. I

6. The method of claim 4 wherein said silicon is added to said powder mixture inthe form of an aluminum-silicon alloy.

7. A method of sintering a superficially oxidized ironaluminum base powder mass consisting essentially by weight of about 4.0% to 11.0% aluminum, not less than 0.5% tin, silicon in an amount not less than about 10.0% by weight of the aluminum present, the remainder being iron, said method comprising progressively heating the powder mixture in a non-oxidizing environment, including a heat treatment at a temperature between about 500 C. and 600 C. for not less than about 1 hour and a final heat treatment at a temperature between about 1300 C. and 1375 C. for not less than about 1 hour.

8. The method of claim 7 wherein said powder mixture is subjected to an intermediate heat treatment at a temperature between about 1050 C. and 1150 C. for not less than about 2 hours.

9. The method of claim 7 wherein said silicon is present in said powder mixture as an aluminum-silicon alloy.

10. A method of preparing an iron-aluminum base alloy by sintering wherein the powder to be sintered consists essentially of about 4.0% to 11.0% aluminum, up to about 15.0% by weight of at least one metal selected from the group consisting of Mo, Nb, Ti, Be, Ni, Cr and Zr, the balance being iron, said method comprising the steps of uniformly admixing with said powder an amount of silicon equal to not less than about 10.0% by weight of the aluminum present, and tin in an amount not less than about 0.5% by weight of the total, pressing the resulting admixture so as to-form a powder compact, and progressively heating the compact in an non oxidizing environment, maintaining it at a temperature between about 500 and 600 C. for not less than about 1 hour. at a temperature between about 1050 C. and 1150 C. for not less than about 2 hours, and terminally at a temperature between about 1300 and 1375 C. for not less than 1 hour.

11. A method of sintering a superficially oxidized ironaluminum base powder mass containing essentially by 'weight about 4.0% to 11.0% of aluminum, a metal additive selected from the group consisting of Mo, Nb, Ti,

. Be, Ni, Cr, and Zr, said additive being present in said mass in an amount not greater than about 15.0%, not

less than about 0.5 tin, and a quantity of silicon such that the silicon to aluminum ratio in said mass in greater than about 0.10, said method comprising the steps heating said mass progressively in an iron-oxidizing environ ment, maintaining it at a temperature between about 500 C. and 600 C. for not less than about 1 hour, and terminally at a temperature between about 1300 C. and 1375 C. for not less than about 1 hour. 7

References Cited in the file of this patent UNITED STATES PATENTS 2,192,742 Howe .Q. Mar. 5, 1940 2,407,234. Guthrie et al. Sept. 10, 1946 

1. A METHOD OF SINTERING A SUPERFICIALLY OXIDIZED IRONALUMINUM BASE POWER MASS CONSISTING ESSENTIALLY BY WEIGHT OF BETWEEN ABOUT 4.0% TO 11.0% OF ALUMINUM, AND THE BALANCE IRON, SAID METHOD COMPRISING THE STEPS OF UNIFORMLY ADMIXING WITH SAID POWDER MASS SILICON IN AN AMOUNT SUCH THAT THE WEIGHT RATIO OF SILICON TO ALUMINUM IS AT LEAST 0.1, AND TIN IN AN AMOUNT EQUAL TO AT LEAST 0.5% BY WEIGHT OF THE TOTAL MIXTURE, AND PROGRESSIVELY HEATING THE RESULTING POWDER MIXTURE IN A NON-OXIDIZING ENVIRONMENT FROM A TEMPERATURE BETWEEN ABOUT 500*C. AND 600*C. MAINTAINED FOR NOT LESS THAN ABOUT 1 HOUR, TO A TERMINAL TEMPERATURE BETWEEN ABOUT 1300*C. AND 1375*C. MAINTAINED FOR NOT LESS THAN ABOUT ONE HOUR. 